This invention relates to forming a solder connection for attaching a gold bump contact of an electronic component to a printed circuit or the like. More particular, this invention relates to an improved method for solder bonding to a gold bump contact utilizing a solder formed of tin-bismuth alloy.
In the manufacture of microelectronic packages, it is known to attach an electronic component to a printed circuit board by soldering to a gold bump contact. For example, a tape automated bonding, a gold bump is affixed to a copper lead on the component. The printed circuit board comprises a copper trace that includes a site, referred to herein as a bond pad, at which the connection is to be formed. Solder, which may be metallic tin or a tin alloy, is applied to the bond pad. With the gold bump superposed onto the bond pad, the assembly is heated and cooled to melt and resolidify the solder to bond the gold bump to the trace. The gold bump connection not only physically attaches the component to the printed circuit board, but also electrically connects the lead of the component to the bond pad on the board for conducting electrical signals to or form the component for processing.
In forming the connection, the assembly is heated to a temperature above the melting point of the tin solder for a time sufficient not only to melt the solder, but also the permit the molten metal to wet the gold surface, which wetting is essential to forming a strong solder bond. Moveover, it is common practice to exceed the melting point by about 40.degree. C. to reduce the time required to complete the the soldering process. Thus, for metallic tin having a melting point of about 232.degree. C., the reflow rate temperatures approach 270.degree. C. For some microelectronic packages, such high temperatures tend to damage electrical features remote from the connection. In manufacturing packages that include temperature sensitive components, gold bump connections may be formed using a solder formed of lower melting tin lead alloy. Tin-bismuth solder alloy is also known for forming lead-free connections.
There is a tendency for gold to dissolved into the tin liquid during reflow and to form tin gold intermetallic phases, which is predominantly AuSn.sub.2, particularly at the higher reflow temperatures. In connections formed with higher melting alloys, the product connection retains sufficient mechanical strength despite the intermetallic phase to permit used within the temperature range normally experienced by such microelectronic packages during operation. However, in connections formed utilizing lower melting tin-bismuth alloy, the formation of the intermetallic phase significantly reduces the mechanical properties of the connection within the revelant operational temperature range. Therefore, there remains a need for a method for forming a connection to a solder bump contact using tin-bismuth solder alloy that reduces the detrimental effect of intermetallic gold tin phases.